This application claims the benefit of the Korean Application No. P2001-0049305 filed on Aug. 16, 2001, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a back light, and more particularly, to a direct type back light for a liquid crystal display (LCD) device.
2. Discussion of the Related Art
Generally, ultra thin sized flat panel displays having a display screen with a thickness of several centimeters or less, and in particular flat panel LCD devices, are widely used in monitors for notebook computers, spacecraft, and aircraft.
Since such an LCD device has no light source in and of itself, it needs a back light provided at the rear of a liquid crystal panel. Such a back light is used as a light source. Generally, as shown in FIG. 1, the LCD device includes a liquid crystal panel 5 comprising first and second substrates 2 and 4, a back light 7 at the rear of the liquid crystal panel 5, and one or more diffusing sheets 6 between the liquid crystal panel 5 and the back light 7.
The back light 7 used as a light source of an LCD includes a cylindrical luminescent lamp. The back light is divided into a direct type and a light-guiding plate type according to the position of the luminescent lamp. A back light is shown, for example, in U.S. Pat. No. 5,835,139, which is hereby incorporated by reference as if fully set forth here.
In the direct type back light, a luminescent lamp is arranged on the whole surface below a liquid crystal panel. To prevent the shape of the luminescent lamp from being displayed on the liquid crystal panel, it is necessary to maintain an adequate distance between the luminescent lamp and the liquid crystal panel. Moreover, a light-scattering mechanism is needed for uniform light distribution. In such case, however, there is a limitation to forming a thin sized back light.
As the size of the panel is enlarged, which is the trend, the light-emitting area of the back light increases. If the direct type back light has a large sized area, the light-scattering mechanism should have a sufficient thickness to make the light-emitting area flat. In this case as well, there is a limitation to forming a thin sized back light.
In the light-guiding plate type back light, a luminescent lamp is provided at a side edge and in the rear of a liquid crystal panel so that light is dispersed to all sides using a light-guiding plate. In this case, since the luminescent lamp is provided at a side edge and light must pass through the light-guiding plate, a problem of low luminance occurs. Also, for uniform distribution of luminous intensity, advanced optical design and processing technologies are required.
As shown, the direct type back light and the light-guiding plate type back light have respective problems. Therefore, a direct type back light is mainly used for an LCD device such as monitors for PC or TV which consider brightness more important than thickness, while a light-guiding plate type back light is mainly used for an LCD device such as notebook computers which consider thickness more important.
A related art direct type back light for an LCD device will be described with reference to the accompanying drawings.
FIG. 2 is a plane view illustrating a related art direct type back light for an LCD device. Referring to FIG. 2, the related art direct type back light includes a plurality of luminescent lamps 1 and an outer case 3 that fixes and supports the luminescent lamps 1. Also, a light-scattering mechanism (not shown) may additionally be arranged over the luminiscent lamps 1.
The light-scattering mechanism prevents a shape of the luminescent lamp from being displayed on a display screen of the liquid crystal display panel and provides a light source having uniform brightness distribution. To enhance the light-scattering effect, a plurality of diffusion sheets or a plurality of diffusion plates may be used.
The outer case 3 supports the luminescent lamp 1 and includes a reflecting plate in an inner side 3a. The reflecting plate is arranged to focus light emitted from the luminescent lamp 1 on the display of the liquid crystal panel, thereby enhancing light efficiency.
The luminescent lamp 1 is called a cold cathode luminescent lamp and acts as a light source of a liquid crystal display device, which emits light when a power source is applied to an electrode 1a arranged between both ends of the lamp.
However, the related art back light for a liquid crystal display device has several problems. A sustain voltage at both ends of the luminescent lamp becomes higher as the length of the luminescent lamp becomes longer. For this reason, a problem arises in that load is applied to an inverter. Also, a problem arises in that ozone (which has an undesirable smell) is generated by reaction between air and a material within the electrode due to high voltage.
To solve such problems, a device has been proposed in which a ground electrode 1b is formed at a middle portion of the luminescent lamp so that a voltage applied to the luminescent lamp can be reduced, as shown in FIG. 3. However, even though both sides around the middle portion have the same length, uniform discharge at both sides is still not actually generated. For this reason, a problem still arises in that luminance is not uniform.